It can be stated in two parts as given below :
(a)    In an atom, no electron pairing takes place in the p, d or for vitals respectively until each orbital of the given sub-shell constrains one electron.
(b)    The unpaired electrons present in the various orbital’s of the same sub-shell should have parallel spins.

For example, each of three p-orbital’s of the 2p-sub-shell gets one electron of parallel spin before any one of them receives a second electron of opposite spin.

Justification of the rule. The presence of the electron each in two orbital’s of the same sub-shell involves laser inter-electronic repulsion as compared to the electrons in the same orbital. Therefore, it leads to lower energy state.

moreover, electron in half-filled orbital’s of the same sub-shell counter less inter electronic repulsion in space when they have parallel spins then when their spins are opposite.

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